A base station in a time-division duplexing (TDD) wireless network uses a high-speed transmit-receive (T/R) switch to alternately couple the antenna to the transmit path during transmit periods and to the receive path during receive periods. Conventional base stations generally use pin-diode switch modules as transmit-receive switches. Circulators have been used in the past for time duplexing of radar signals, but are generally not used in base stations.
FIG. 1A illustrates selected portions of conventional base station 100A according to one embodiment of the prior art. Base station 100A comprises time-division duplexing (TDD) transceiver (X-CVR) 110, pin-diode switch module 115, and antenna 150. TDD transceiver 110 comprises a transmit path and a receive path. The transmit path of TDD transceiver 110 comprises transmit (XMIT) circuitry 111, power amplifier (PA) 112, low-noise amplifier (LNA) 121, and receive (RCV) circuitry 122.
Pin-diode switch module 115 comprises capacitor 125, inductor 130, capacitor 135, pin-diode 140, capacitor 145, quarter-wave (λ/4) transmission line 155, pin diode 160, and capacitor 165. A T/R switch control signal from a base transceiver subsystem (BTS) controller (not shown) is coupled to a filtered bias line (i.e., capacitor 135 and inductor 130) that turns pin-diode 140 on during transmission and off during reception. During transmit periods, quarter-wave transmission line 155 and pin-diode 160 present a high impedance (or open circuit) to the transmit signal from power amplifier 112. During receive periods, quarter-wave transmission line 155 and the off-state (open) pin-diode 160 in parallel with the LNA input impedance (e.g., 50 ohms) present a 50 ohm impedance to the receive signal from antenna 150.
Unfortunately, however, pin-diode 140 in the transmitter path is the cause of high transmitter insertion loss (e.g., 0.75 dB to 1.5 dB) and high levels of harmonics and third-order inter-modulation products. The harmonics and inter-modulation products are caused by the non-linear characteristics of pin-diode 140.
FIG. 1B illustrates selected portions of a conventional radar system according to an alternate embodiment of the prior art. Radar system 100B comprises transmitter 180, receiver 185, circulator 190 and antenna 195. Circulator 190 has three ports, namely Port 1, Port 2 and Port 3. Signals entering one of the ports follow the circling arrow and exit at the next port.
Thus, in the ideal case, transmit signals from transmitter 180 enter Port 1 and are emitted at Port 2 to antenna 195, and no portion of the transmit signal is emitted at Port 3 to receiver 185. Also, in the ideal case, receive signals from antenna 195 enter Port 2 and are emitted at Port 3 to receiver 185, and no portion of the receive signal is emitted at Port 1 to transmitter 180.
However, a circulator by itself cannot protect the receiver input in the case of high antenna voltage standing wave ratio (VSWR). A high VSWR antenna condition reflects part or all of the transmit signal-power directly into receiver 185, which can damage the low-noise amplifier. A secondary problem is that, even though there is no transmit signal during receive mode, the output of transmitter 180 has substantial noise present due mostly to high transmitter gain. This noise can reflect off antenna 195, enter receiver 185, and desensitize receiver 185 due to decreased signal-to-noise ratio (SNR). Thus, the circulator configuration in FIG. 1B is not a good solution for high power transmitters, unless it is certain that antenna VSWR is low prior to transmitting.
Therefore, there is a need in the art for improved base stations for use in time-division duplexing (TDD) wireless networks. In particular, there is a need for an improved transmit-receive switch for use in a base transceiver subsystem of a TDD wireless network.